In general, optical semiconductor devices are constructed by resin sealing of an optical semiconductor element such as a light emitting diode or photodiode with an epoxy resin composition. In the case of blue LED and white LED which have attracted the attention in recent years, the sealing resin greatly requires heat resistance during electrification which can endure even under heat generation, in addition to optical transparency and light resistance.
When a conventional epoxy resin composition is used as the sealing resin, heat and light resistance becomes insufficient, and deterioration of the luminance is caused in a short period of time. Thus, in Patent Document 1 and Patent Document 2, for example, a study on use of the following alicyclic epoxy resin is disclosed as means for improving the light resistance.

However, according to the techniques in which such an alicyclic epoxy resin is used, heat resistant transparency is still insufficient, and further improvement of the heat resistant transparency is strongly desired.
In addition, in Patent Document 3, for example, a study on use of an epoxy resin composition containing the following silicone resin having epoxy groups is disclosed as a sealing resin for optical semiconductors.

However, the polyether structure produced by cation polymerization of the epoxy group is generally inferior in the heat resistance, therefore, cured products of the curable composition having many epoxy groups are accompanied problems of heat and light resistance. Accordingly, there still exists a great need for development of a heat and light resistant resin having optical transparency.    [Patent Document 1] Japanese Unexamined Patent Application No. 2003-292568    [Patent Document 2] Japanese Unexamined Patent Application No. 2001-19742    [Patent Document 3] Japanese Unexamined Patent Application No. 2004-238589